mod_vmd — Voicemail Beep Detection

mod_vmd is a dialplan application module (src/mod/applications/mod_vmd) that detects the characteristic beep tone played by a voicemail system at the end of its greeting. It uses the Discrete Energy Separation Algorithm 2 (DESA-2) to estimate the instantaneous frequency and amplitude of each audio frame, looking for a sustained tone in the 600–1100 Hz range. Detection runs in O(1) time per frame with no external library dependencies.

Operators use this module when originating calls that may land on a voicemail box and need to know precisely when to begin recording a message. When a beep is confirmed, the module sets a channel variable and fires a custom event so the dialplan or an ESL application can react immediately.

Note: The FreeSWITCH source logs a deprecation notice at CRITICAL log level (SWITCH_LOG_CRIT) at load time and directs operators to migrate to mod_avmd, which is the actively maintained successor.

Loading the Module

The module name is mod_vmd. It is not present in the default vanilla autoload_configs/modules.conf.xml and must be added manually:

<load module="mod_vmd"/>

No external libraries or special build flags are required; the module compiles as part of the standard FreeSWITCH build tree.

Configuration

mod_vmd ships no .conf.xml file and registers no XML configuration parameters. All tuning is performed through channel variables set before the application or API command is invoked (see Channel Variables below).

Dialplan Applications

Application	Purpose	Arguments
vmd	Start or stop voicemail beep detection on the current channel	[start] | stop

vmd

Attaches a read-leg media bug to the session that continuously analyses incoming audio. The optional argument start (or no argument) begins detection; passing stop removes the bug and ends detection. Running a second start on a channel that already has the bug active logs a warning and returns without error.

When a beep is confirmed the module:

1. Sets the channel variable vmd_detect to TRUE.
2. Fires a CUSTOM event with subclass vmd::beep.

Example dialplan usage:

<extension name="vmdtest">
  <condition field="destination_number" expression="^vmd(\d{10})$">
    <action application="answer"/>
    <action application="vmd"/>
    <action application="sleep" data="25000"/>
    <!-- vmd_detect will be TRUE here if a beep was heard -->
    <action application="info"/>
    <action application="hangup"/>
  </condition>
</extension>

To stop detection mid-call:

<action application="vmd" data="stop"/>

API Commands

Command	Purpose	Syntax
vmd	Start or stop detection on an arbitrary session by UUID	vmd <uuid> <start|stop>

vmd

The API form lets an ESL client or mod_event_socket script control detection on a live session without requiring dialplan access.

vmd 5ec74a35-e9e4-42cb-8a0d-d68e5e146bab start
vmd 5ec74a35-e9e4-42cb-8a0d-d68e5e146bab stop

Returns +OK on success or -USAGE: <uuid> <command> on invalid input.

Channel Variables

Variable	Set / Read	Purpose
vmd_detect	Set	Set to TRUE by the module when a voicemail beep is confirmed. Readable by the dialplan after a sleep or bridge returns.
vmd_min_time	Read	Override the minimum sustained-beep duration (in DESA-2 sample units; default 8000). Read once when the vmd dialplan application starts. Not read when detection is started via the vmd <uuid> start API command — the API always uses the compiled-in default of 8000.

Events

mod_vmd fires one custom event subclass:

vmd::beep (SWITCH_EVENT_CUSTOM) — fired when a confirmed beep ends. Headers included:

Header	Value
Beep-Status	stop
Beep-Time	Integer — duration of the beep in DESA-2 point units
Frequency	Estimated beep frequency in Hz
Unique-ID	UUID of the originating session
call-command	vmd

The event is both queued to the session and fired to the global event system.

Source: src/mod/applications/mod_vmd